The present invention relates to braided tubular structures, processes and machines for manufacturing them, and their applications in the construction of plane armorings for composite material.
European Patent EP-A-0 113 196 has already disclosed a braided (and nonwoven) tubular structure, which is intended to form a composite tubular part after impregnation with resin and polymerization of the latter. This tubular structure includes longitudinal threads (parallel to the axis of said structure) and helicoidal braiding threads forming two grids of directions oblique in relation to said longitudinal threads, said braiding threads being interlaced with one another, as well as with said longitudinal threads. These longitudinal threads are distributed on a plurality of collars coaxial with the structure and the helicoidal braiding threads follow paths which cause them to pass between the longitudinal threads of one or of a plurality of concentric collars, these paths being such that the projections of said braiding threads onto a plane orthogonal to the axis of the tubular structure are broken lines, all the sections of which are oblique in relation to the thickness of the wall of said tubular structure.
It is possible in this way to obtain braided tubular structures of considerable wall thickness. However, in spite of this, the mechanical wall characteristics of the tubular composite parts constructed from these tubular structures are relatively low, since these tubular structures exhibit numerous voids due to the intersections of the paths of the helicoidal threads of one and the same direction.
The object of the present invention is to remedy this disadvantage.
To this end, according to the invention, the braided tubular structure including longitudinal elongate elements distributed on a plurality of collars coaxial with the structure and braiding threads forming two grids of directions oblique in relation to said longitudinal elongate elements and interlaced with said longitudinal elongate elements, said braiding threads following paths which cause them to pass between said longitudinal elongate elements, is noteworthy in that said braiding threads of each one of said grids form an assembly of superposed layers in which said braiding threads are parallel from one layer to the next.
Thus, said parallel braiding threads, which form layers which are superposed in the direction of the thickness of the wall of said tubular structure, permit the minimization of the intersections and thus the imparting of excellent mechanical properties to said wall.
Said longitudinal elongate elements may be threads as in the European Patent identified hereinabove. They may likewise be composed of thread rovings, of cables, of rods, of tubes, etc . . .
In order to be able to construct such a tubular structure, it is advantageous, in accordance with the present invention, to braid it by means of a plurality of braiding threads which are wound off from so many spools which are displaceable through a succession of elementary paths that said braiding threads should become interlaced with elongate elements which are parallel to one another and distributed on a plurality of coaxial collars. According to the invention, this process is noteworthy in that said spools are displaced with the aid of three types of elementary paths, that is to say first elementary paths of direction orthogonal in relation to said coaxial collars, second elementary paths of direction circumferential in relation to the latter and third elementary paths, each one of which associates a first and a second elementary path inter se.
Thus, said first elementary paths permit said braiding threads to be caused to pass around said elongate elements of different collars, that is to say within the thickness of the wall of said tubular structure, while said circumferential second paths permit the constitution of said superposed layers formed of threads which are parallel from one layer to the next.
It will be noted that, by varying the sequence of said elementary paths, it is possible to vary the braiding xe2x80x9cmeshxe2x80x9d. Since numerous sequences of different elementary paths are possible, by virtue of the invention, it is possible to obtain a large number of different xe2x80x9cmeshesxe2x80x9d for the tubular structure of the present invention.
For the implementation of such a process, the present invention advantageously provides a braiding machine including a set of identical multiindented wheels, mounted on a fixed support and driven in rotation to cause the circulation of spindles carrying spools of braiding threads, said multiindented wheels being disposed in parallel and equidistant lines. In order to be capable of executing the three types of elementary paths which are characteristic of said process, this machine is, according to the invention, noteworthy:
in that, in said set of multiindented wheels:
the multiindented wheels belonging to a line are spaced from one another, so that two consecutive multiindented wheels belonging to one and the same line cannot cooperate directly with one another;
a multiindented wheel belonging to one line is disposed between two multiindented wheels belonging to an adjacent line and cooperates with said two multiindented wheels; and
in that, in the spaces between the consecutive multiindented wheels of a line, there are provided passages for said longitudinal elongate elements of said braided tubular structure.
The result is accordingly a quincuncial arrangement for the multiindented wheels and for said passages, said quincunces of multiindented wheels being overlapped with said quincunces of said passages. Within such quincunces, the distance between the centers of two consecutive multiindented wheels of a line is equal to the distance between two consecutive passages of a line, these distances being equal to twice the distance between two adjacent lines of multiindented wheels and of passages.
By virtue of such a quincuncial arrangement, it is possible to cause the circulation of said spindles transversely and parallel to said lines of multiindented wheels, and also to cause them to turn about the center of a multiindented wheel, so that it is possible to execute the three types of elementary paths mentioned hereinabove.
To do this, according to another feature of the present invention, in said spaces between the consecutive multiindented wheels of a line, there are further provided rhomboidal needles of controlled orientation which are intended to direct said spindles to cause them to pass from one multiindented wheel to another belonging to an adjacent line (first and second elementary paths) or to maintain them on their respective multiindented wheels (third elementary paths).
Preferably, said needles are coaxial with said passages for the elongate elements, so that they likewise form quincunces, which are superposed upon those of said passages.
Said needles can be controlled in orientation by a linkage gear.
Preferably, at each instant:
all the needles associated with a line of multiindented wheels are parallel to one another and thus have a common orientation; and
the common orientations of the needles associated with two adjacent lines of multiindented wheels are symmetrical with respect to one another in relation to said lines.
Said fixed support of the multiindented wheels may be plane. Nevertheless, for reasons associated with braiding mesh clamping, it is advantageous that said plane support should be cylindrical. In this case, said plane support may be constituted by the internal wall of a cylindrical collar. Such a collar may have its axis disposed vertically or horizontally.
Moreover, French Patents FR-A-2 610 951 and FR-A-2 610 952 have already disclosed a plane woven armoring for composite material including weft threads distributed on a plurality of superposed levels and warp threads passing around weft threads situated at different levels. Thus, such an armoring exhibits high characteristics of delamination. However, on account of the fact that threads which make up said armoring follow, in a plane, only two directions (the warp direction and the weft direction), the composite parts which incorporate it may, for certain applications, exhibit an insufficient shear modulus, parallel to said plane.
Thus, the object of the present invention is also to construct such a plane armoring having an improved planar shear modulus.
To this end, according to the invention, the plane armoring for composite material including parallel elongate elements distributed on a plurality of superposed levels, in the manner of weft threads, is noteworthy:
in that it includes:
a first grid of threads which are parallel to one another, passing around said elongate elements situated at different levels;
a second grid of threads which are parallel to one another, passing around said elongate elements situated at different levels; and
in that the general directions of the threads of said first and second grids are symmetrical with respect to one another in relation to said parallel elongate elements.
According to the present invention, to obtain such a plane armoring:
initially there is constructed a braided tubular structure including parallel longitudinal elongate elements distributed on a plurality of collars coaxial with the structure and braiding threads which form two grids of directions oblique and symmetrical in relation to the longitudinal elongate elements and interlaced with said longitudinal elongate elements, said braiding threads following paths which cause them to pass between said longitudinal elongate elements, so that said braiding threads of each one of said grids form an assembly of superposed layers in which said braiding threads are parallel from one layer to the next; then
said tubular structure is split along a longitudinal cutting line parallel to said longitudinal elongate elements.
Thus, after the longitudinal cutting of said tubular structure and flattening of the latter, the result obtained is a plane armoring as mentioned hereinabove, in which, on the one hand, said first and second grids of parallel threads are constituted respectively by said grids of braiding threads and, on the other hand, said parallel elongate elements distributed on a plurality of superposed levels are formed by said longitudinal elongate elements distributed on the plurality of coaxial collars of said braided tubular structure.
Thus, such a process permits the construction, by braiding, that is to say at high speed, of the plane structure according to the invention. The latter can therefore be manufactured more rapidly than the conventional woven armorings, although it exhibits great improvements as compared with the latter.
It will be noted that, to obtain such a plane armoring, it is advantageous to make use of the braiding machine specified hereinabove, equipped with cutting means to split longitudinally the braid formed by said machine.